1. Field of the Invention
The present invention relates in general to a method of producing a piston for a compressor which compresses a gas, and more particularly to a method of producing such a piston having a hollow cylindrical head portion which is slidably fitted in a cylinder bore formed in a cylinder block of the compressor.
2. Discussion of the Related Art
A piston used for a compressor is reciprocated within a cylinder bore formed in a cylinder block of the compressor. In view of this, it is desirable to reduce the weight of the piston. When the piton is used for a swash plate type compressor, in particular for a variable capacity type swash plate compressor, it is particularly required to reduce its weight. As the swash plate type compressor for compressing a refrigerant gas used in an air conditioning system of an automotive vehicle, the above-described variable capacity type swash plate compressor has been recently used, wherein the angle of inclination of the swash plate with respect to a plane perpendicular to the axis of rotation of the drive shaft is variable to change the discharge capacity of the compressor. In the swash plate type compressor for the vehicle, it is generally required to increase a rotation speed of the drive shaft for thereby attaining an improved operating performance of the compressor, so as to meet the demand for reducing the size of the compressor. To this end, it is necessary to reduce the weight of the piston. In the variable capacity type swash plate compressor wherein the angle of inclination of the swash plate is adjusted on the basis of a difference between the pressures in a compressing chamber which is partially defined by the piston, and a crank chamber in which the swash plate is disposed, it is particularly required to reduce the weight of the piston for achieving a stable adjustment of the inclination angle of the swash plate and reducing the noise of the compressor during its operation.
The assignee of the present invention proposes, in the Japanese Patent Publication No. 9-105380 and its corresponding U.S. Pat. No. 5,174,728, a technique of reducing the weight of the piston used for the variable capacity type swash plate compressor. Namely, the piston having a hollow head portion which is slidably fitted in the cylinder bore is produced, by first preparing a hollow cylindrical head member having an open end and a closed end, then closing the open end of the head member by a closure member which is formed integrally with an engaging portion which engages the swash plate, and finally welding the head member and the closure member together. The head member and the closure member are both formed by forging.
The formation of the head member and the closure member by forging inevitably pushes up a cost of manufacture of the piston. To reduce the cost, the piston with a hollow head portion is formed by die-casting. In the die-cast piston, however, it is difficult to weld the head member and the closure member to each other, and this problem makes the die-cast piston unsuitable for practical use. Further, it is desirable to reduce the weight of the piston in other types of the compressor such as a fixed capacity type as well as the variable capacity type.
It is therefore an object of the present invention to provide a method of producing a piston for a compressor, wherein at least a hollow cylindrical head portion of the piston is formed by die-casting and a closure member is welded to the hollow cylindrical head portion.
The above object may be achieved according to any one of the following forms or modes of the present invention, each of which is numbered like the appended claims and depends from the other form or forms, where appropriate, to indicate and clarify possible combinations of technical features of the present invention, for easier understanding of the invention. It is to be understood that the present invention is not limited to the technical features and their combinations described below. It is also to be understood that any technical feature described below in combination with other technical features may be a subject matter of the present invention, independently of those other technical features.
(1) According to a first feature of the present invention, there is provided a method of producing a piston for a compressor, comprising the steps of casting a molten material into a die to form a hollow cylindrical head portion of the piston that is open at one of opposite ends thereof and is closed at the other end thereof, such that an amount of gas included in the material of the head portion is not more than 5 cc per 100 g of the material; closing, with a closure member, the open end of the head portion; and welding the head portion and the closure member to each other by emitting, a plurality of times, a welding beam toward each of a multiplicity of spots on a welding line along which the head portion and the closure member contact each other.
In the present production method, it is desirable that the closure member be also formed by die-casting. However, this is not essentially required. The piston may include an engaging portion which is integrally formed with the closed end of the hollow cylindrical head portion and which engages a drive mechanism, and the closure member may be a cover member which just closes the open end of the head portion. In this particular case, the cover member may be produced by a method other than the die-cast method; such as machining of a commercially available common material, such as a bar-like material, or forging. It is desirable that the head portion and the closure member be formed of an aluminum alloy that has a small specific gravity and is easily die-cast. However, the head portion and the closure member may be formed of a different material such as a magnesium alloy.
In a conventional die-cast piston, the amount of gas included in the material of the hollow cylindrical head portion of the piston is 10 to 30 cc per 100 g of,the material under normal condition, i.e., one atmospheric pressure and room temperature. In contrast thereto, in a piston produced by a special die-cast, such as pore-free method or forging cast method, described later, the amount of gas included in the material of the hollow cylindrical head portion of the piston can be lowered to 5 cc per 100 g of the material udder the normal condition. The open end of the head portion is closed with the closure member, and the head portion and the closure member are welded to each other by emitting a welding beam, two or more times, toward each of a number of spots on a welding line along which the head portion and the closure member contact each other, so that respective welded portions of the head portion and the closure member have only a small amount of blowholes. The thus produced piston is suitable for practical use. The welding beam, such as an electronic beam or a laser beam, may be emitted to form a beam spot on the welding line, and one of the welding beam and the combination of the head portion and the closure member may be rotated relative to the other to move the beam spot on the welding line. Thus, respective welding portions of the head portion and the closure member, in the vicinity of the welding line, are molten and bonded to each other. At this Time, the gas included in the material of the members are heated and expanded to run away into the atmosphere, so that blowholes are produced in the welding beads. However, since the amount of pas included in the material is at the low level of not more than 5 cc per 100 g of the material, the amount of the blowholes produced is also at a low level. In addition, the blowholes which arc once produced in the welding beads are closed when the welding beads are subjected to another exposure to the welding beam and are molten, so that the welding beads have a still less amount of blowholes. The thus produced die-cast piston is highly suitable for practical use The phrase of xe2x80x9cemitting, a plurality of times, a welding beam toward each of a multiplicity of spots on a welding linexe2x80x9d is defined as meaning emitting, a plurality of times, a welding beam to each of a multiplicity of points on a welding line or the vicinity of the each point. That is, according to the present invention, it is not required to emit, a plurality of times, a welding beam to, strictly, each of a multiplicity of points on a welding line, but it is possible to emit, for the first time, a welding beam to each of a multiplicity of points on a welding line and, for the second time, the welding beam or another welding beam to substantially the same point as the each point.
According to the present feature, the amount of gas included in the material of the head portion is not more than 5 cc per 100 g of the material, more preferably not more than 3 cc per 100 g of the material, most preferably not more than 1 cc per 100 g of the material.
(2) According to a second feature of the present invention that includes the first feature (1), the step of casting the molten material comprises casting the molten material in a pore-free die-cast method.
The pore-free die-cast method is defined as a casting method in which a molten metal such as an aluminum alloy is cast into a cavity of a die (e.g., a movable die and a stationary die) in the state in which the cavity is filled with an active gas such as oxygen and, since a high degree of vacuum is produced in the cavity because of the reaction of the molten metal and the active gas, the gas is prevented from being included or involved into the material of the cast product. The cast product enjoys a high strength even if the thickness of wall thereof may be small.
(3) According to a third feature of the present invention that includes the first feature (1), the step of casting the molten material comprises casting the molten material in a forging-cast method.
The forging-cast method is defined as a casting method in which a high pressure, e.g., 30 to 200 MPa, is applied to a fully or half molten metal which has been cast in a die and this state is kept until the molten metal is solidified. This method can reduce the amount of gas that is included or involved into the material of the cast product.
(4) According to a fourth feature of the present invention that includes any one of the first to third features (1) to (3), the step of welding the head portion and the closure member comprises welding the head portion and the closure member to each other by emitting each of a plurality of welding beams toward the each of the spots on the welding line while moving at least one of the each of the welding beams and a combination of the head portion and the closure member relative to the other of the each of the welding beams and the combination, so that the each of the spots is exposed to the each of the welding beams.
In the welding step according to the fourth feature (4), at least one of (a) the respective proportions of respective intensities of the plurality of welding beams and (b) the distance between the plurality of welding beams (e.g., two welding beams) may be so changed that in the state in which the welding beads which are produced by a prior one of the welding beams have been solidified to an appropriate degree, the welding beams are exposed to a subsequent one of the welding bees. Once the proportion of intensity of each of the welding beams is determined, it is possible to determine an appropriate speed at which the each welding beam is moved to melt appropriately the metal materials on both sides of the welding line. Meanwhile, in order to eliminate appropriately the blowholes, it is desirable, as indicated above, that in the state in which the welding beads which are produced by a prior one of the welding beams have been solidified to an appropriate degree, the welding beams are exposed to a subsequent one of the welding beams. To meet both of those requirements, it is effective to change at least one of (a) the respective proportions of respective intensities of the plurality of welding beams and (b) the distance between the plurality of welding beams.
(5) According to a fifth feature of the present invention that includes the fourth feature (4), the step of emitting the each of the welding beams comprises oscillating at least one of the welding beams, relative to the welding line, while moving the one welding beam relative to the combination.
It is otherwise possible to move simply at least one of the welding beams relative to the combination. However, it is effective to oscillate at least one of the welding beams, relative to the welding line, in order to eliminate the blowholes and/or increase the strength of the welding.
(6) According to a sixth feature of the present invention that includes the fifth feature (5), the step of oscillating the one welding beam comprises rotating the one welding beam so as to describe a conical surface.
At least one of the welding beans may be moved along the welding line relative to the combination while being iteratively rotated to describe a conical surface. In this case, the welding spot formed by the one welding spot formed by the one welding beam describes a locus, shown in FIG. 8, along the welding line, so that the amount of blowholes is reduced. It is speculated that this effect would result from the fact that each of the welding beads is molten two or more times.
(7) According to a seventh feature of the present invention that includes any one of the first to third features (1) to (3), the step of welding the head portion and the closure member comprises welding the head portion and the closure member to each other by emitting at least one welding beam toward the each of the spots on the welding line while moving at least one of the one welding beam and a combination of the head portion and the closure member relative to the other of the one welding beam and the combination, so that the each of the spots is exposed, the plurality of times, to the one welding beam.
(8) According to an eighth feature of the present invention that includes the seventh feature (7), the step of emitting the one welding beam comprises oscillating the one welding beam relative to the welding line while moving the one welding beam relative to the combination.
(9) According to a ninth feature of the present invention that includes any one of the first to eighth features (1) to (8), the step of welding the head portion and the closure member comprises welding the head portion and the closure member to each other under a predetermined welding condition which assures that respective portions of the head portion and the closure member that are molten by the welding beam at a last time of the plurality of times are contained in a sum of respective portions of the head portion and the closure member that are molten by the welding beam at each time of the plurality of times that is prior to the last time. The phrase xe2x80x9csum of respective portions of the head portion and the closure member that are molten by the welding beam at each time of the plurality of times that is prior to the last timexe2x80x9d means any portion of the head portion and the closure member that has been molten at least one time prior to the last time. For example, in the case where the position of the center of the welding spot formed by the welding beam emitted at a second time differs from that at a first time, the portion molten at the second time may be offset from the portion molten at the first time, in a direction perpendicular to the welding line. If the portion or portions molten at the last time is or are fully contained in the sum of the portions molten at the first and second times, the predetermined welding condition according to the ninth feature (9) is satisfied. In other words, at the last time, the welding beam must not melt any new portion of the head portion or the closure member.
(10) According to a tenth feature of the present invention that includes the ninth feature (9), the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at each subsequent time of the plurality of times are contained in respective portions of the head portion and the closure member that are molten by the welding beam at each prior time of the plurality of times that is prior to the each subsequent time.
In the welding step according to the tenth feature (10), the welding beam emitted at each time subsequent to the first time does not melt any new portion of the head portion and the closure member. That is, the portion or portions molten at each subsequent time coincides with, or are smaller than, the portion or portions molten at each prior time. If the time duration between two successive times is short, the temperature of the respective portions of the head portion and the closure member that are molten at the prior or first time is usually still high when those portions are exposed to the welding beam at the subsequent or second time. Therefore, if the subsequent exposure is effected under the same condition as that under which the prior exposure is effected, the predetermined welding condition according to the tenth feature is not satisfied. That is, the condition for the subsequent time must be more moderate than that for the prior time.
(11) According to an eleventh feature of the present invention that includes the ninth or tenth feature (9) or (10), the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at the last time are contained in respective portions of the head portion and the closure member that are molten by the welding beam at a first time of the plurality of times that is prior to the last time.
(12) According to a twelfth feature of the present invention that includes any one of the ninth to eleventh features (9) to (11), the step of welding the head portion and the closure member to each other comprises emitting, at least three times including at least one time between a first time and the last time, the welding beam toward the each of the spots on the welding line, and wherein the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at each of the at least one time are contained in respective portions of the head portion and the closure member that are molten by the welding beam at the first time.
(13) According to a thirteenth feature of the present invention that includes any one of the ninth to twelfth features (9) to (12), the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at the last time coincide with respective portions of the head portion and the closure member that are molten by the welding beam at least one time of the plurality of times that precedes the last time. So long as the present feature is concerned, there are no times between the one time that precedes the last time, and the last time.
(14) According to a fourteenth feature of the present invention that includes any one of the ninth to thirteenth features (9) to (13), the predetermined welding condition comprises that a speed at which at least one of the welding beam and a combination of the head portion and the closure member is moved relative to the other of the welding beam and the combination, at the last time, is equal to a speed at which the one of the welding beam and the combination is moved relative to the other of the welding beam and the combination, at least one time of the plurality of times that precedes the last time, and that an intensity with which the welding beam is emitted at the last time is equal to an intensity with which the welding beam is emitted at the at least one time.
(15) According to a fifteenth feature of the present invention that includes any one of the ninth to fourteenth features (9) to (14), the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at each subsequent time of the plurality of times are contained in respective portions of the head portion and the closure member that are molten by the welding beam in each prior time of the plurality of times that is prior to the each subsequent time, and the predetermined welding condition comprises at least one of (a) that a speed at which at least one of the welding beam and a combination of the head portion and the closure member is moved relative to the other of the welding beam and the combination, at the each subsequent time, is not lower than a speed at which the one of the welding beam and the combination is moved relative to the other of the welding beam and the combination, at the each prior time, and (b) that an intensity with which the welding beam is emitted at the each subsequent time is not higher than an intensity with which the welding beam is emitted at the each prior time.
The phrase xe2x80x9cnot lower thanxe2x80x9d means both the case of xe2x80x94equal toxe2x80x94and the case ofxe2x80x94higher thanxe2x80x94, but the case of xe2x80x94higher thanxe2x80x94is preferable to the case ofxe2x80x94equal toxe2x80x94. Similarly, the phrase xe2x80x9cnot higher thanxe2x80x9d means both the case ofxe2x80x94equal toxe2x80x94and the case ofxe2x80x94lower thanxe2x80x94, but the case ofxe2x80x94lower thanxe2x80x94is preferable to the case ofxe2x80x94equal toxe2x80x94.
The phrase xe2x80x9ceach subsequent timexe2x80x9d used in the fifteenth feature (15) means, for example, the last time used in the ninth or eleventh feature (9) or (11), all times subsequent to the first time, used in the tenth feature (10), or the last time and the at least one time between the first and last times, used in the twelfth feature (12). The xe2x80x9ceach prior timexe2x80x9d used in the feature (15) means, for example, all times prior to the last time, used in the ninth or tenth feature (9) or (10), or the first time used in the eleventh or twelfth feature (11) or (12). That is, the phrase xe2x80x9ceach subsequent timexe2x80x9d may mean a subsequent one of each pair of successive times, and the phrase xe2x80x9ceach prior timexe2x80x9d may mean a prior one of each pair of successive times. In addition, the phrase xe2x80x9ceach subsequent timexe2x80x9d may mean one or more times subsequent to a certain time, and the phrase xe2x80x9ceach prior timexe2x80x9d may mean one or more times prior to a certain time. These are true with the sixteenth feature (16) described below.
(16) According to a sixteenth feature of the present invention that includes any one of the ninth to fifteenth features (9) to (15), the predetermined welding condition assures that respective portions of the head portion and the closure member that are molten by the welding beam at each subsequent time of the plurality of times are contained in respective portions of the head portion and the closure member that are molten by the welding beam at each prior time of the plurality of times that is prior to the each subsequent time, and the predetermined welding condition comprises that an amount of oscillation of the welding beam relative to the welding line at the each subsequent time is less than an amount of oscillation of the welding beam relative to the welding line at the each prior time.